1. Field of the Invention
This invention relates to the field windings of dynamoelectric machines, and more particularly, to means for embedding the end turns of such field winding in the dynamoelectric machines rotor and restraining such winding in the rotor by use of wedges.
2. Description of the Prior Art
Conventional dynamoelectric machine rotor construction provided for distributing a field winding associated with such rotor within longitudinally-extending slots formed on the outer periphery of the rotor. The field winding portions situated in the longitudinally-extending slots were often restrained therein by means of retainer wedge bars which, when inserted in the longitudinal slots, closed the mouth of the longitudinal slots with the wedge bars often being held in place by means of a dovetail or tapered construction which mated with the walls of the longitudinal rotor slots.
At each axial end of the rotor, the field winding exited the longitudinal slots and passed circumferentially along the periphery of the rotor until the appropriate longitudinal slot was intersected. At such time, the field winding was directed into an appropriate longitudinal slot located on the opposite circumferential side of the rotor pole being wound. Such winding process was continued with the longitudinal slots circumferentially nearer the rotor pole being filled prior to winding the outer longitudinal slots situated about the pole. Thus, for each complete winding pass around each of the rotor poles, the field winding was deformed on each axial end to make the turn-arounds to the return longitudinal slot. Such turn-arounds are known as field winding end turns and were not typically supported by conventional retainer wedge bars. However, due to the high centrifugal forces exerted on the field winding end turns during rotor rotation a constraining device called a retaining ring or retaining sleeve was often utilized. The retaining ring was frequently heated and then shrunk into place on each axial end of the rotor about the radial periphery of the end turns. As generator ratings have increased in recent years, so have generator rotor diameters and strength requirements of the associated retaining rings. Retaining ring strength was often enhanced by increasing its outside diameter since its inside diameter was dependent on rotor diameter. As the outside radial dimension of the retaining ring increased, so did the self-induced centrifugal forces acting thereon. Thus, increasing the radial thickness of retaining rings results in an interaction between opposing effects: augmentation of retaining ring strength due to an increase in its cross-sectional, stressed area and reduction of its strength due to the increase in centrifugal forces exerted thereon.
The outside diameter of such retaining ring or sleeve structures now constitute a serious limitation to further increases in rotor diameter size. Such increases in rotor diameter size are necessary for developing machines of increased ratings. New developments of high-strength materials may permit an increase in retaining ring outside diameter and thus rotor size, but such developments are speculative and unpredictable and, thus, cannot be relied upon to provide retaining rings which permit future, significant increases in dynamoelectric machine ratings.
U.S. Pat. No. 3,634,709, which issued Jan. 11, 1972, illustrates one approach for eliminating the retainer ring. The end turn portions of the field winding are embedded in circumferential slots formed in the rotor and are restrained therein by a double layer helical wire winding applied circumferentially about longitudinally disposed wedges which, on their radially inner side, contact the end turn portions. Utilizing such wire wrapping requires formation of longitudinal wedges whose axial end regions are of complex shape which can be costly to manufacture and difficult to assemble. Furthermore, the aforementioned patent requires additional plates be assembled with the longitudinal wedges to lock the wire wrapping in place. French Pat. No. 2,145,103 illustrates a cryogenic alternator having a continuous slot disposed about each rotor pole. Due to the slot's continuous, non-intersecting character, retainer bars must be welded therein rather than hermetically inserted to restrain the rotor's field windings. Such continuous slots were judged to be difficult to manufacture and not amenable to mechanical insertion of wedges therein for restraining field windings disposed within those slots. Thus, although the prior art avoids some of the problems presented by retaining rings, it introduces others.